Let $A$ be a subset of the cyclic group $\mathbf{ Z}/p\mathbf{ Z}$ with $p$ prime. It is a well-studied problem to determine how small $|A|$ can be if there is no unique sum in $A+A$, meaning that for every two elements $a_1,a_2\in A$, there exist $a_1',a_2'\in A$ such that $a_1+a_2=a_1'+a_2'$ and $\{ a_1,a_2\}\neq \{ a_1',a_2'\}$. Let $m(p)$ be the size of a smallest subset of $\mathbf{ Z}/p\mathbf{ Z}$ with no unique sum. The previous best known bounds are $\log p \ll m(p)\ll \sqrt{ p}$. In this paper we improve both the upper and lower bounds to $\omega(p)\log p \leqslant m(p)\ll (\log p)^2$ for some function $\omega(p)$ which tends to infinity as $p\to \infty$. In particular, this shows that for any $B\subset \mathbf{ Z}/p\mathbf{ Z}$ of size $|B|<\omega(p)\log p$, its sumset $B+B$ contains a unique sum. We also obtain corresponding bounds on the size of the smallest subset of a general Abelian group having no unique sum.
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